Strange CNG leakage problem

I'm having a problem with my CNG valve leaking - but only at times. What is happening is that the CNG valve in my car (or something thereabouts) seems to start leaking gas right after filling the tank up fully - but this happens only sometimes, usually after I've just refilled CNG, and the problem seems to go away soon after.

When it does happen, I can hear a loud hissing noise from the area the valve is in, but this noise decreases over a couple of hours or so, and then disappears altogether after some more time. I've tried locating the leak with the soapy water test, but have had no luck - no bubbles form in any of the usual areas where leaks usually occur. And the noise stops altogether after a while, which doesn't make sense - if there was a leakge, the noise shouldn't stop altogether?

What's even stranger is that the noise disappears altogether after a while, and the amount of gas in the cylinder doesn't seem to have gone down by any appreciable amount, at least if I'm going by the mileage I get. On average a full tank of CNG gives me around 200km without A/C (possibly 10-20 km more, since I've never ran the cylinder completely dry) and between 170-180 odd kms with A/C. And bear in mind that this is city mileage. So since I'm getting fairly decent mileage (similar to what I've been getting all along), the valve obviously isn't leaking most of the gas out, since I'd be getting significantly lower mileage if it was.

As usual, most of the car mechanics are no help whatsoever - most of them just tinker around with the CNG system and proclaim the problem "fixed", but no-one really seems to know what's going on. I'm wondering if there's a problem at the pump I usually refill at - maybe they're pumping more gas in, and the excess is escaping?? Or is this a problem with my setup, despite the fact that the "soapy water" test reveals nothing?? Or is it just some excess pressure escaping? Not sure, but the problem sure has me foxed.

If anyone has faced a similar problem, or has any ideas on whats going on, I'm all ears!

Thanks,

Rahul

Edit: My car is a Maruti Zen 99 model - I got CNG installed in it last year. Real happy with the mileage, but seems to be one thing after the other that keeps on going "wrong" with it, and very few, if any, mechanics in the Delhi NCR region seem to understand whats going on. . .

if any of you is in Delhi NCR region goto any cng repair centre they should be able to rectify if not then goto JMD autogas in gurgaon or Greater Noida they will certainlly be able to rectify the problem.

I am not familiar with the CNG system. Having said that, I believe that there should be a valve for regulating the excess gas. It will vent the excess gas and when the pressure returns to normal, shut down. You will not be able to find any leaks once the pressure comes down to normal. But if you test it for leaks while the noise is there, you will find this valve venting gas. The reason why there is no difference in mileage noticed is because the excess gas escapes quickly, leaving the normal quantum which accounts for your FE. And this will remain constant. This will explain Rahul's issue. If you have the same issue, then there is nothing to worry. However, to be sure, get it checked.

Mee too, have observed the similar problem. After filling up the tank fully, within a short span of first 1 hour, I observed, 3 points of GAS is getting leaked, even if the vehicle is kept without running !

This problem started after 1.5 years of usage, and the Maruti Service Center team says, they don't have equipment to test the GAS leakage with full pressure.

Truly disappointed, as this problem is really a big hole in my pocket !

Please update, if anybody is able to get the resolution for this problem.

If in doubt go to any CNG mechanic & get your filling valve changed & recheck, if you can't find CNG shop that can help then PM me I will arrange filling valve & get your car checked ( Iam from west delhi)

I faced the same issue, there are usually 2 different type of sounds are coming after getting the CNG filled. One is the slower which seems to coming from valve but the second type of voice is very strong like a warning beep sound and usually comes when accelerator is pressed.

Is anyone have idea for the reason of this issue and how this can be resolved?